This invention pertains to combination drill and drill adapters and, in particular, to metal cutting drills fitted into threaded shank adapters for high torque applications.
Twist drills for metal cutting and other applications are normally made of appropriate steel or other alloys, such as high speed tool steels. For normal applications, the drill shank (the end opposite the drill cutting tip) is of cylindrical configuration for mounting into a conventional chuck of a drilling machine which consists of a plurality of jaws which tighten around the drill shank to hold it securely in place. In certain high torque applications, the tight fit between the chuck and the drill shank is insufficient to prevent the drill shank from rotating relative to the chuck. Drill shank adapters have been utilized for ease of changing, weight and space considerations, but have not solved the problem of slippage. Drill shank adapters in the past have had low torque capabilities and, consequently, their use has been restricted primarily to aluminum applications.
One conventional manner of mating of the drill shank and adapter has been by brazing. The temperature needed to melt conventional brazing alloys can be 800.degree. F. or more, depending on the brazing alloy and drill and adapter materials employed. Because the drill is normally made of a heat treated steel or other alloy, these temperatures can cause a marked change and deterioration in the properties of both the drill bit and the threaded shank adapters (including the threaded areas for mating into the chuck). Such temperatures may result in undesired tempering of the drill shank and/or the cutting edges of the drills, which lowers the hardness and cutting properties of the drill, and has an adverse impact upon the metallurgical and mechanical properties of the shank adapters. Stress relieving or other heat treatment of the tools may be employed after the brazing process to avoid some of the aforementioned problems, but such treatment may also weaken the brazing bond since it may be carried out near the melting point of the brazing alloy.
Furthermore, brazing and subsequent heat treatments add cost and complexity to the drill assembly operations and may have a detrimental effect upon the safety of workers and environmental considerations as a whole. Also, the brazing and other heat treatments leave a dull finish, which is not the desirable condition when used to drill certain materials such as aluminum.
Press or interference fitting of the drill bit shank into the adapter hole is an alternative to brazing. When the degree of interference fit (i.e., the diameter mismatch between the drill shank diameter and the adapter hole diameter) and the depth of penetration of the drill shank into the adapter hole are carefully selected, it may be possible to produce a threaded shank drill assembly in the smaller sizes with only minor straightening adjustment required after fitting. However, for drill bits of larger sizes, e.g., drill diameter larger than No. 30 (0.1285 in.), press or interference fitting is generally less successful in that torque specifications as defined by National Aerospace Standard (NAS) 965 could not be met consistently.
Additionally, if the interference was increased by making the drill shank considerably larger than the adapter hole to enhance torque strength, the drill bits and adapters became bent and many of the threaded shank adapters split or ruptured. Furthermore, the excessive press loads required to successfully press the drill shank into the threaded shank adapter to meet the higher torque specifications can cause misalignment of the press head on the adapting machine and yield a threaded shank drill assembly with extreme axial misalignment (runout).
In view of the aforementioned problems, it is therefore an object of the present invention to provide a combination drill and drill adapter which is particularly suited for high torque applications in drilling metals and other materials.
It is another object of the present invention to provide a combination drill and drill adapter which may be assembled without causing damage and deterioration of material properties normally encountered as a result of the high temperatures employed in brazing.
It is a further object of the present invention to provide an improved threaded shank drill which can be assembled without damage to the drill bit or adapter.
It is yet another object of the present invention to provide a high torque drill adapter in which both the drill bit and adapter retain high hardness levels.
It is a further object of the present invention to provide a process for fitting a drill bit into a drill adapter which avoids damage to the machinery for making the finished tool.
It is another object of the present invention to provide a non-brazing alternative to press or interference fitting of drill bits into shank adapters which avoids damage to the tool and its components.
It is a further object of the present invention to provide a threaded shank drill adapter with reduced runout.